1. Field of the Invention
This invention relates to ceramic and polymer/ceramic composites having a porous network wherein the macro-shape of the composite, microarchitecture of the network, porosity and the pore sizes in the network can be simultaneously and precisely controlled or designed. This invention further relates to methods for producing such composites by stereolithography and the varied uses thereof.
2. Related Art
Micro-architecture and pore size are extremely important for osteoconduction in ceramic bone grafts. All of our organs (such as liver, bone, and kidney) constitute a "Parenchyma," which is the physiologically active component, and the "Stroma," which is the framework that supports the organization of the parenchyma. In soft tissue, loss of parenchyma with maintenance of stroma allows a remarkable degree of regeneration and repair. Thus to design an implant for osteoconduction it is desirable to mimic the architecture of the interstitial or stromal bone. An idealized bone graft substitute would mimic osteon-evacuated bone and have an interconnected porous system of channels of similar dimension. Many in vivo studies have revealed the significance of the porous structure on the promotion of bone growth.
The fabrication of such osteon-evacuated complex architecture or controlled microarchitecture and controlled porosity ceramic and ceramic/polymer composite structures is difficult and has been the subject of many investigations. See, for example, Fabbri et al., Biomaterials 16 (1995) pp. 225 et seq; Liu, "Fabrication of Hydroxyapatite Ceramic with Controlled Porosity," J. Mater. Res.Mater. Med. 8 (1997) pp. 227-232; Arita et al., J.Mater.Sci.Mater. Med. 6 (1995) pp. 19; and Fabbri et al., "Granulates Based on Calcium Phosphates with Controlled Morphology and Porosity for Medical Applications," Biomaterials 6 (1994) pp.474-477.
Classical studies by Hulbert et al., "Potential of Ceramic Materials as Permanently Implantable Skeletal Prostheses," J. Biomed. Mater. Res. 4(1970) pp.433-456, as early as 1971established minimum pore size requirements of 100 microns for successful bone in-growth. Klein et al., "Macroporous Calcium Phosphate Bioceramics in Dog Femora: A Histological Study of Interface and Biodegradation," Biomaterials 10 (1989) pp.59-62, studied sintered HAp material containing pores of 150-250 microns in dog femurs. The volume fraction porosity and pore interconnectivity was not disclosed. Constantino, et al., "Hydroxyapatite Cement: I. Basic Chemistry and Histologic Properties," Arch. Otolaryngol. Hecul. Neck Surg. 117 (1991) pp. 379-384, studied HAp cement material that had a volume fraction porosity of 10% and 20%. The pore dimensions and connectivity were not reported. The ideal pore dimensions and the porosity of the interconnected porous system for osteo-conduction has still not been determined or achieved.
Currently, there is one product available with a porous infrastructure similar to that of bone. This structure is made by a process named "replaminform" which utilizes the skeletal structure of marine invertebrates, especially reef building corals, as a template to make porous structures. Roy et al., "Hydroxyapatite Formed From Coral Skeletal Carbonate by Hydrothermal Exchange," Nature 247 (1974) pp. 220-222. Two species of coral having a suitable pore size were identified for replicating into hydroxyapatite (HAp)and used as a bone substitute. Holmes. et al., "A Coralline Hydroxyapatite Bone Graft Substitute," Clin. Orthop. 188 (1984) pp. 252-262. To mimic the osteon evacuated stroma of cortical bone, the coral skeleton from the genus Porites was selected. To mimic the cancellous bone, the genus Gonioporo was selected. Hanusiac, "Polymeric Replaminform Biomaterials and A New Membrane Structure," (Ph.D. thesis, Pennsylvania State University, 1977). However, porositys attained by natural corals, though reproducing that of tissues, appear questionable owing to the nature of the walls which contain impurities which are able to disturb the reaction mechanisms between ceramics and hard tissues. Osborn, "Hydroxylapatite Kermik granulate und ihro slatematik," Zeitschrift Material (1989) pp 2-12. In addition, Corralline HAp materials must still be formed into the desired macroscopic shape for implantation.
None of the current fabrication techniques enable the osteon-evacuated micro-architecture of the bone to be duplicated and none of these techniques allow the fabrication of a predetermined, tailored macro and micro-architecture.
Porous ceramics have been produced in many configurations and pore sizes using many processing routes. The most common approach for producing porous ceramic is via the replication of a polymeric porous structure. The process involves impregnation of a foam structure with ceramic materials by immersing it into a ceramic slip. The subsequent firing of the resultant structure pyrolyzes the substrate and sinters the ceramic powder. Porous HAp ceramics have been fabricated by Fabbri. et al. by impregnating cellulose sponge structures (interconnected macro-pores &gt;150 microns) with HAp slurry. The main disadvantages of these replicas are there is no control of the architecture and the structures have inferior mechanical properties which limits their load bearing clinical applications.
Another approach to forming a porous ceramic is by incorporating volatile or combustible phase or phases that are lost during firing. This approach has been utilized for the fabrication of porous HAp ceramics using poly vinyl butyral as a porosifier, see Liu.
Another approach to forming ceramic porous structures is by the foaming of ceramic slurries. This involves incorporating a gaseous phase dispersed into a ceramic suspension. The suspension typically contains the ceramic material, water, a polymeric binder, a surfactant, and a gelling agent. Arita et al. obtained porous HAp ceramic sheets by means of a tape casting technique with CaCO.sub.3 as a gas-forming agent. HAp ceramic sheets with a highly porous microstructure (up to 62%) were successfully developed. However, the pore size was limited to only several microns.
Solid free form fabrication (SFF) is a new manufacturing technology also known as rapid prototyping or layered manufacturing. Various SFF machines like stereolithography (SL), selective laser sintering (SLS), fused deposition modeling (FDM), laminated object manufacturing (LOM), and three-dimensional Printing (3d printing) are now commercially available. Recently, SFF has been used for medical prototyping. In medical applications, a physical model serves as a hard copy of the data set, illustrating the shape, orientation, relative location and size of internal anatomical structures for diagnosis, operation planning, design of implants, external and internal prostheses, surgical templates, and communication and teaching. See, McAloon, "Rapid Prototyping: A Unique Approach to Diagnosis and Planning of Medical Procedures," SME, Dearborn, Mich. (1997). The 3d printing process has been used for making polymeric medical devices. See Cima et al., "Fabrication of medical devices by solid free form fabrication," U.S. Pat. No. 5,490,962 (1996). Similarly SLS process has also been used for making bone prosthesis. See Fink. et al., "Rapid, Customized bone Prosthesis," U.S. Pat. No. 5,370,692 (1994).
The technique of stereolithography (SL) has been widely used for rapid prototyping of polymeric parts. See, Jacobs, "Rapid Prototyping and Manufacturing: Fundamentals of Stereolithography (Society of Manufacturing Engineers, Dearborn, Mich., 1993). The SFF method of SL has advantages over other SFF method in fabricating precision parts and high quality ceramic/polymer composite structures. The process of the current invention aims at fabricating end use structures for various applications rather than model prototypes by using biocompatible organics and inorganics in the SL process.